Use code behind or place markup and code in single file?

There is no performance difference between code-behind and single-page model. Both models are functionally the same and execute the same way at run time

Which model you should use depends on you and your organization's needs.

Advantages of single-file: easier to deploy one file instead of two, easier to rename page, easier to study because you have all in one file etc.

Advantages of code-behind: clean separation of user interface and code, designer and programmer can work together at the same time, designer can't see the code, code can be used for more than one page etc.

Generally, if you have a lot of code or you need to separate page design from coding, you should use code-behind